It is well known that thin metallic films can be used to convert a portion of microwave energy incident thereon to thermal energy and that such thermal energy may be used to crispen and brown foodstuff heated by microwave energy, as described, for example, in U.S. Pat. No. 4,641,005. The metal most commonly used is aluminum.
Aluminized polymeric substrate films having the required thickness of aluminum thereon are commercially available. It is often desirable, when such films are used for the generation of thermal energy, for the metal to be located only on a portion of the substrate surface.
There have previously described procedures for effecting selective demetallization of the substrate by etching away metal from the undesired regions of the polymeric film substrate. These procedures are described in U.S. Pat. Nos. 4,398,994, 4,552,614, 4,610,755 and 4,685,997, the disclosures of which are incorporated herein by reference. By removing metal from selected portions of the polymeric substrate, thermal energy is generated only from the remaining adhered metal.